184 HYDRATES IN AQUEOUS SOLUTION. 



a strictly linear scale can not correctly register all the wave-lengths of a grat- 

 ing spectrum. Nevertheless, the grating belonging to the spectrograph was 

 of such short focus that for the region of wave-lengths of the first-order 

 spectrum used, the error just mentioned was negligible in comparison with 

 those explained below. 



It is a well-known fact that celluloid negative films, after development, 

 fixing, and drying, differ appreciably in length from the same linear dimen- 

 sion which they possessed before being so treated. If all the films of one 

 make would behave quantitatively in exactly the same way, the variations 

 in length to which attention has just been called, would be of no importance 

 from the present point of view. But films of the same kind change by very 

 different amounts, and this phenomenon is inconvenient, to say the least. 

 Therefore, to have linear scales to fit accurately each and every one of the 

 complete spectrograms, it would be necessary to make as many different 

 scales as there are different distances between the extreme spectral lines, 

 say, throughout the entire series of spectrograms. Obviously, to do this 

 would entail an amount of labor that would be utterly disproportionate 

 to the gain in accuracy finally obtained. Consequently, only four or five 

 negative scales of different total length were obtained by photographic 

 production from the same paper scale, which was ruled and numbered for 

 the present work. Since the negative scales cover the entire range of vari- 

 ation in length of the spectrograms, the wave-lengths of selected lines, when 

 read off from the scales on the published plates, ought not to contain any 

 large error. 



In making the positives, the scale line numbered 48 was placed opposite 

 to the corresponding strong emission line of the comparison spectrum. The 



o 



latter line has a wave-length of about 4,800. 1 Angstrom units, and is desig- 

 nated by 73 on plate 1 (a) . It is probable that in a few cases the two lines 

 just named, the one on the negative scale and the other on the negative 

 spectrogram, were displaced a little with respect to each other in the process 

 of printing. 



To avoid having the lines of the numbered scale too close together, each 



O 



of the smallest intervals denotes 25 Angstrom units. 



The wave-lengths of the numbered lines of plate 1 (a)* were derived from 

 the two following sources: 



"An Introduction to the Study of Spectrum Analysis," by W. M. Watts. 



"Measurements of the Wave-Lengths of Lines of High Refrangibility 

 in the Spectra of Elementary Substances. " Hartley and Adeney. From 

 the Philosophical Transactions of the Royal Society, Part I, 1884. 



* The negative was not a single exposure. To stand reproduction the extreme ultra- 

 violet was " favored." 



